1. Field of the Invention
The present invention relates to a method for fabricating porous UO2 sintered pellets for recovering metallic nuclear fuel, and a fabrication method thereof.
2. Description of the Related Art
Spent nuclear fuel (UO2) from a light water reactor (LWR) generally includes fissile material (U) that is not consumed, and transuranic elements (TRU) that are generated from the burning. Along with this, UO2 also includes fission products. The pyroprocess is a recycling technology implemented to produce metallic nuclear fuel for use in a fast reactor, through pyrometallurgical and electrochemical processing from irradiated UO2 fuel in the LWR, thus providing advantages including good nuclear proliferation resistance. To recover the fissile material, the pyroprocessing mainly includes a pretreatment process to fabricate UO2 sintered pellets from U3O8 powder, and a follow-up process to convert the fabricated UO2 sintered pellets (i.e., ceramic nuclear fuel) into metallic nuclear fuel. The presence of fission products is desirably removed in the pretreatment process an consideration of the considerable influence on the follow-up process where the ceramic fuel is converted into metallic fuel.
To be specific, the pretreatment process generally involves the disassembly/cutting of a fuel rod, decladding, compacting, and sintering, and the follow-up process mainly involves an electrolytic reduction, electro-refining, and electro-winning. The decladding in the pretreatment process relates to extracting spent UO2 sintered pellets from the disassembly/cut fuel rod, in which the UO2 sintered pellets within the fuel rod are generally converted into U3O8 in an air atmosphere at temperatures ranging between 350 and 700° C. The UO2 pellets are powdered owing to a volume expansion in accordance with the decreased density, and thus escape from the fuel rod. As the phase changes from UO2 pellets to U3O8 powder from oxidation, gaseous volatile fission products including iodine (I) and bromine (Br) existing in the pellet are vaporized.
After the decladding, the U3O8 powder is compacted into the desired shapes and dimensions using a compacting machine such as a press. Then, by sintering at the appropriate temperature under the desired atmospheric gas (e.g., oxidising, inert, nitrogen, and reducing atmospheric gas), porous sintered pellets are fabricated, and are suitable for a volatilization of the fission products and are suitable for handling. Porous UO2 sintered pellets are advantageous, considering the fact that fission products are easily vaporised, and when the following electrolytic reduction is processed with UO2 rather than U3O8, the O/U ratio is decreased from 2.67 to 2.00, and owing to the decrease in the existing oxygen, the processing efficiency is increased greatly. Further, the process yield is increased, such that there is an advantage of increased productivity.
Korean Patent No. 10-0293482, incorporated herein by reference in its entirety, teaches a method for fabricating UO2 sintered pellets, which includes steps of fabricating green pellets by adding various kinds of sintering aids into oxidized U3O8 powder transformed from UO2 spent nuclear fuel, and fabricating UO2 sintered pellets by sintering the green pellets at temperatures above or equal to 1500° C. in a reducing atmosphere, thereby providing the advantage of providing UO2 sintered pellets with high sintered density.
Further, Korean Patent No. 10-1020783 discloses a method for fabricating porous granule from spent nuclear fuel, which includes steps of preparing fine U3O8 powder by charging the spent nuclear fuel into voloxidizer apparatus, where the fuel is rotated and annealed under oxidation condition at 450° C. to 600° C., preparing U3O8 powder and metallic oxides by additionally annealing the fine U3O8 powder and the metallic fission products while rotating these under an oxidation condition of 700° C. to 800° C., and preparing porous UO2+z granule by additionally annealing U3O8 powder under an inactive condition of 1000° C. to 1300° C. while rotating the U3O8 powder.
Meanwhile, conventionally, porous UO2 sintered pellets for an electrolytic reduction for recovering metallic fuel is in a cylindrical shape, and such cylindrical sintered pellets require sufficient time to ensure that fission products are volatized during atmospheric sintering and reduction.
Further, for the collar-shaped UO2 sintered pellets to be electro-reduced into metallic fuel (U & TRU), it is necessary that molten salt, which is one of the electrolytic substances, infiltrate into the UO2 sintered pellets through the pores existing on the surface thereof. It takes a considerable amount of time until the molten salts infiltrate into the sintered pellet, and difficulty of molten salt infiltration into an interior of the pellets during an electrolytic reduction often causes imperfect an electrolytic reduction efficiency.
Accordingly, to prevent the shortcomings mentioned above, the UO2 sintered pellets are sized to have loss diameter and longer length. However, this requires more caution in the process of fabricating and handling the sintered pellets, and even affects throughput and economic aspect.
Additionally, it is reported that uranium dioxide (UO2), the spent nuclear fuel of the pressurised water reactor (PWR), has a varying burnup distribution depending on the length thereof, even in the same fuel assembly and fuel rod. That is, the burnup at the beginning and ending portions of the fuel rod are relatively smaller, while the middle portion has foe greater burnup. Such differences in the burnup distribution cause an irregular distribution of the contents depending on the composition of the fissile material (i.e. U, TRU) generated within nuclear fuel, and fission product, and can also influence the microstructure of the nuclear fuel.
The size of the grains of the microstructure of the nuclear fuel varies depending on the burnup or linear heat racing, and based on one burnup and in the direction from the edge to the center axis, the UO2 sintered pellets can be divided ante an undistributed area before irradiation, i.e., an equiaxed area and a columnar area. The varying particle sizes depending on the burnup also influences the particle size of the oxidized U3O8 powder, and the particle size of the oxidized powder also tends to increase, when the burnup increases.
Such difference in burnups, i.e., the inhomogeneous distribution of TRU concentration containing plutonium and irregular particle since, can affect the accuracy in the metric measurement of nuclear fuel substances, which can greatly contribute to nuclear transparency. Without an accurate measurement of nuclear fuel substances, it would be difficult to achieve stability of processing factors and reproducibility in the fabrication of porous UO2 sintered pellets.
Accordingly, while continuing with studies on a method for fabricating porous UO2 sintered pellets for an electrolytic reduction, the inventors of the present invention were able to develop a method for fabricating porous UO2 sintered pellets, which can resolve the problems occurring in the prior art, and thus completed the present invention.